Why does my wood start burning halfway through a cut?

That darkening halfway through is almost always resin gumming up the flutes, not the bit going dull mid-job. Pine is full of pitch, and once the bit warms up the soft stuff melts and sticks to the cutting edges. From there the bit stops shearing the wood cleanly and starts rubbing, which makes more heat, which sticks more resin. It snowballs fast, which is exactly why the first few minutes looked great and then went downhill with zero changes to your settings.

Pull the bit out and look at it under a light. If the flutes have a shiny amber or black film on them, that's your answer. A quick soak in rubbing alcohol or a citrus cleaner and a scrub with an old toothbrush gets it off, and you'll notice the difference imediately on the next cut. The fix going forward is mostly about keeping the chips flying. On a little 3030-class machine the temptation is to feed slow because it feels safer, but slow feed with high RPM is exactly the rubbing condition you're describing. You generaly want to either bump the feedrate up or drop the spindle speed so each tooth takes an actual bite. Light dust-colored powder coming off means you're rubbing. Nice little chips or granules means you're cutting. Also try shallower passes with a faster feed rather than one deep slow pass. The enclosure makes it worse, yeah. Limited airflow means the bit and the workpiece stay hot, and hot pine dumps resin way more readily. Even a small fan moving air through the box helps a fair bit. And blow the dust off the bit and the pocket between passes if you can, since chips sitting in the cut reheat and redeposit that gunk.

Some darkening on pine isn't a disaster and plenty of it sands out, but that glossy scorched look means you're burning, not just tinting. Get the pitch off the bit, feed it a bit harder, and you should see it mostly disappear.

I ran into somthing similar with some leftover plywood last month, ended up being sap baked onto the spinning part too. Freezing the bit for a bit before the cut made cleanup way easier for me, no idea if that's a real trick or just luck. Does pine sap come off with plain dish soap or do I need the stronger stuff the other poster mentioned? Also curious how often you all clean yours, mine seemed grimy after one job.

Same deal here with some maple cutting boards a while back, except no enclosure at all and it still burned. What acutally fixed it for me was a cheap single-flute upcut bit meant for plastics, the sharp edge just shaved the material instead of pressing it. Wiped clean after every cut, barely any pitch buildup. Might be worth trying once you've cleaned what you've got. Also curious about your dust setup, are you running any extraction at all or just blowing it out by hand between parts?